An officiating team for a tennis match is composed of up to twelve personnel, of whom ten serve as court line-judges. It may seem excessive to find two players surrounded by ten line-judges on the court, but careful examination of the line-judging task readily explains why so many judges are needed. With players using state of the art racquet technology, a tennis ball may travel as fast as 50 meters per second. At this velocity the human eye and brain are not agile enough to respond and determine the exact location of the ball's impact on the ground. An official can know for certain whether the ball has bounced inside or outside the court only he is seated along the particular court line for which he is responsible. Since there are up to ten court lines to officiate, important matches are usually manned with that many line-judges. Due to the particularly high velocity of a service hit, moreover, faultless base line officiating of the service zone, even with all this personnel, is not possible without the assistance of an auxiliary apparatus.
Many ways to replace line-judges have been suggested:
U.S. Pat. No. 4,866,414 removes the burden of human line-judging by assigning optical line watch units to each of the court lines, but it places part of the units on the court itself and requires the players to wear socks and shoes of non-white colors.
U.S. Pat. No. 5,059,944 also describes an optical system for detecting and signaling a ball-out-of-bounds condition on a tennis court. In this patent, an optical shape-plane interfered with by a ball disables an underlying optical timed plane. The timed plane discriminates between an interference caused by a player's body or racquet and one caused by a ball. There is, however, no visual feedback on the system, and in any case, it is not precise enough for reliable officiating.
U.S. Pat. No. 4,422,647 suggests a way of detecting and indicating that a volleyball is out of bounds, utilizing a light beam system that distinguishes between the volleyball and other beam disrupting agents. The deficiency of this patent seems to be that the timer requires exact simultaneous interruption of both beams to prevent the system from indicating an out-of-bounds condition.
U.S. Pat. No. 4,814,986 describes a device for monitoring the relative point of impact of an object in flight proximate to a reference line on a surface. At least one plane of radiated energy, preferably light beams, is pre-positioned with respect to the reference line. Detectors, preferably photodetectors, provide signals indicative of the relative elevation of the object at two successive points in time-based intersection of at least one plane of energy radiated by the object. The system does not, however, provide a means of differentiating between ball and player or ball and racquet. It is therefore only applicable for officiating of the service zone.
U.S. Pat. No. 4,718,669 offers an electrically operated line monitor for tennis. It uses one or more rays substantially smaller than a ball in effective cross section, in order to monitor areas adjacent to critical lines of a court. As with U.S. Pat. No. 4,814,986, cited above, however, this invention does not provide a means of differentiating between ball and player or ball and racquet, and is likewise only applicable for officiating the service zone.
U.S. Pat. No. 5,082,263 claims a tennis ball that contains a radar signal reflecting element, so that a computer can compare a radar signal, sent to and received from the ball, with a stored position of the court, thereby determining the relative position of the ball. The obvious drawback of this invention is the required change in the design of the ball.
U.S. Pat. No. 4,432,058 describes an automated tennis officiating system that analyzes effects of the bounce of the ball. This patent utilizes pattern recognition in order to determine whether the detected bounce was actually a valid bounce of a ball and not caused by other events. The invention has two drawbacks: the court must be laid out with electrical circuits that define "in" and "out" areas of the court, and the ball must be electrically conductive.
U.S. Pat. No. 4,893,182 claims an imaging processing system for displaying a succession of selected separate images of a moving object, but in an otherwise substantially static scene, like a bowling alley. Such a patent is not applicable to a tennis game.
U.S. Pat. No. 4,545,576 describes an apparatus and method to compute the trajectory of a moving object by remote, non-interfering sensors. The particular application, based on video cameras, computes the trajectory of a pitched baseball throughout its flight. The apparatus is required, among others things, to identify the ball and compute its location in three dimensions as a function of time. The precision that can be achieved in computing the position of the ball, however, is not sufficient for the game of tennis.
Finally, U.S. Pat. No. 4,797,738 claims a color recognition apparatus that uses a video image, represented by a color difference signal and photographed by a color television camera or similar device. The apparatus makes a determination whether or not these signals fall within a predetermined region. Natural variation in illumination of the ball is problematic, however, and regardless, the precision is insufficient in this patent as well.
As described above, all of the prior art solutions remain wanting in some respect. Some are quite cumbersome and require substantial changes in the playing court or tennis ball. Others impose restrictions on the players' clothing. Still others cannot handle the dynamics of a modern tennis game. In short, none of them are fully satisfactory for use with current conventionally acceptable professional practice.
The present invention, in contrast, provides a fully satisfactory system for automatic line officiating. It is operated by a single person, the referee, and it can determine the exact point on the ground where the ball has bounced.